The major magnetoelectronic device families are current-in-plane (CIP) spin valves, current-perpendicular-to-the-plane (CPP) spin valves, and magnetic tunnel junctions (MTJ). The lateral spin valve (LSV) is a minor device family. These devices have found technological success in at least two categories of applications. The first category is described as magnetic field sensors. The primary application is the sensor in a read-back head for magnetic media storage systems, such as hard disk drives (HDD). Other applications include position sensors and motion sensors. The second category is described as integrated electronics. The primary application is the storage element in a nonvolatile magnetic random access memory (MRAM). Other applications include nonvolatile switches for reprogrammable logic circuits.
Thin ferromagnetic film devices are generally of the form F1/N/F2. Such devices are often called magnetoelectronic devices. F1 and F2 are ferromagnetic materials, and each can be a single layer or a multilayer. N can be a nonmagnetic material (typically a metal), in which case the device is a spin valve, or it can be a low transmission barrier (typically a dielectric tunnel barrier), in which case it is a magnetic tunnel junction. The devices have two dominant applications: (i) a magnetic field sensor, as used in the read-back head in a magnetic media hard disk drive; (ii) a storage cell in an integrated, nonvolatile magnetic random access memory (MRAM).
The area on a chip that is used by a device is given in units of the minimum lithographic feature size, f. For example, with few exceptions (discussed below in reference to prior art), all the magnetoelectronic devices have a planar geometry. Spin valves and MTJs use a sandwich geometry. The ferromagnetic (F), nonmagnetic and/or dielectric tunnel barrier layers (N) are fabricated as a sandwich in the plane of the wafer. The chip area of these devices is given by the product of the lateral dimensions, the length and width of the sandwich. For example, a CPP spin valve or MTJ using a sandwich shaped like an ellipse and having width 1f and length 2f would have an area on the chip of 2f2. Leads attach the sandwich using the top and bottom surfaces and do not add additional area. A CIP spin valve might require additional area for attaching the leads; thus, such a device might have an area on the chip of 4f2. The lateral spin valve is fabricated with the ferromagnetic and nonmagnetic material layers in the plane of the wafer, but the layers are spatially separated and are not in registration as would be the case for a sandwich geometry. Typically F1 and F2 are fabricated with width f and separated by f, and additional length 2f is required to attach leads. It follows that a typical lateral spin valve might use an area of 5f2 on a chip surface.
Geometry
For convenience, the coordinate axes for a chip can be labeled such that the surface is the xz plane and y is normal to the surface of the chip. Typically, the devices are built up as layered structures with each layer on top of the other. The layers are usually thin films with relatively small thickness in the y direction, but having lateral extent in the xz plane. A common convention is to treat the layer as nearly two dimensional, and to say that the plane of the layer is the xz plane. For the lateral spin valve all layers are in the xz plane, but are not stacked in registration, one upon the other. Spin valves and MTJs have a sandwich structure in which each layer is stacked, in registration, upon another layer.
FIG. 1A is a perspective view of a prior art magnetic tunnel junction 100. The figure shows a rectangular sandwich structure with shape anisotropy ratio of 2:1. More generally, the devices have the shape of an ellipse and the shape anisotropy may vary from 1:1 to 5:1. FIG. 1A could also depict a prior art CPP spin valve if the “middle” layer 110 is taken to be a thin nonmagnetic metal film.
Operation
Magnetoelectronic devices are generally considered to be magnetoresistive devices. The structure can be characterized by an electric resistance, and the value of the resistance differs according to the two different conditions (device states) in which the magnetizations M1 and M2 of F1 and F2 are either parallel or anti-parallel. Typically the device state with M1 and M2 parallel has low resistance R, and the state with M1 and M2 anti-parallel has high resistance R+ΔR. The relative change of resistance is called the magnetoresistance ratio, or simply the magnetoresistance, MR=ΔR/R. Note that the lateral spin valve has the opposite convention: the state with M1 and M2 parallel has relatively high resistance R+RS, the state with M1 and M2 anti-parallel has relatively low resistance R−RS, and R is a baseline resistance with a value determined by details of the geometry. Also note that typically one of the magnetizations is pinned in a fixed direction. This is called the pinned layer. The second magnetization is called the free layer. The magnetization states described as parallel or anti-parallel are now determined by the orientation only of the free layer.
The device parameters are typically (i) applied bias current, (ii) measured voltage, and (iii) external magnetic field. In other cases, (i) a bias voltage is supplied, (ii) a current is measured, and the magnetization of the free layer may be controlled (iii) by applying a spin polarized current to the free layer, instead of using an external magnetic field.
Two device families are recognized and are distinguished by the flow direction of bias current (or, less common, voltage). In a prior art current-in-plane (CIP) spin valve, the F1/N/F2 spin valve is typically fabricated with all layers in the xz plane and stacked in a sandwich geometry. The bias current is applied such that it flows in the xz plane, for example along x, and the resistance is measured along x. In a current-perpendicular-to-the-plane (CPP) spin valve, the spin valve is typically fabricated with all layers in the xz plane and stacked in a sandwich geometry. The bias current is applied such that it flows perpendicular to the xz plane, along y, and the resistance is measured along y, for example from the top to the bottom of the sandwich stack. The MTJ is a CPP device. A bias current is supplied by applying a bias voltage from the bottom electrode (e.g., F1) to the top (F2). The tunnel barrier has a high electrical resistance, the dominant voltage drop is across the barrier, and the bias current is driven to flow with a direction perpendicular to the barrier and therefore perpendicular to the F1 and F2 layers.
Magnetization Manipulation
For sensor applications, fringe magnetic fields from bits recorded as domains in magnetic media change the magnetization orientation of the free layer and the device resistance then varies.
For MRAM, the free layer typically has a uniaxial anisotropy axis parallel to the axis of magnetization of the pinned layer 120. The free layer 130 is designed to have two stable magnetization states, parallel or anti-parallel with the magnetization orientation of the pinned layer 120, and the two different device resistances represent the two binary states, 0 and 1. The storage cell is designed to operate in an integrated way, and two separate mechanisms, Oersted Switching and Spin Torque Switching, are used to manipulate the magnetization orientation of the free layer 130.
In Oersted Switching, two integrated thin film wires (write wires) can be fabricated to be proximal with the device (and, in particular, with the free F layer 130). Current pulses in these wires are inductively coupled to the device, thereby applying pulses of local magnetic field. For appropriate magnitudes, these field pulses superpose in a way that sets the magnetization state of the free layer 130 into a given state, while that of the pinned layer 120 remains fixed. Reversing the polarity of the “write current” pulses will set the magnetization of the free layer 130 into the opposite state. Since the memory cells are formed in a two-dimensional array, the write wires are also formed as a two dimensional array. Write wires pass along rows or columns of cells. Pulses in two write wires that intersect at a given cell uniquely set the magnetization state of that cell, while other cells along the row or column are not affected. This is called “half-select” switching. “Toggle switching” is a pulse sequence that is more complicated, but also relies on Oersted switching with a two dimensional array of write wires.
In Spin Torque Switching, a spin polarized current driven into a thin, patterned ferromagnetic film applies a torque on the magnetization. Pulses of spin polarized current, with positive or negative polarity, can be used to set the magnetization of the free layer 130 into one or the other state.
Edge Junctions
Tunnel junctions are commonly fabricated between two superconductors. In the field of superconductivity, a structure with two superconducting films, e.g. in the xz plane, and a tunnel junction between the two films is called a planar junction. It is also common to fabricate a tunnel junction that contacts one of the films at the film edge. In other words, one superconducting film (S1) is taken to have lateral extent in the xz plane and thickness along y. An insulating layer can be fabricated on the top surface of S1. One edge of S1 can be “cleaned,” and a second film S2 can be fabricated, also in the xz plane, overlapping the edge of S1. S1 and S2 are next to each other, contiguous in the xz plane, and typically a portion of S2 is on top of the insulating layer that coats the top surface of S1. This kind of tunnel junction is called an edge junction.
Magnetic Edge Junctions
Some prior art references describes tunnel junctions fabricated using the edge of one of the ferromagnetic layers, e.g. F1. These references describe magnetic tunnel junctions used for a sensor application. Similar to the edge junction used with superconducting films, F1 and F2 are next to each other, contiguous in the xz plane, and typically a portion of F2 is on top of the layer that coats the top surface of F1. In prior art references, this is called a gap layer, and is typically an insulating material (see e.g., FIG. 1B). FIG. 1B is a cross-section view of a prior art device. The references describe a sensor that has F2 in the middle, and has two tunnel junctions, one on each of the left and right edges. F1 is typically two separate ferromagnetic films, one overlapping the right edge of F2 and the other overlapping the left edge of F2. These devices are described as CIP structures: Film F2 is in the xz plane and both parts of F1 are in the xz plane, except for the small portions that overlap onto the top of the insulating layer above F2. These portions carry no current. Instead, the current is applied at one end of F2, travels in the plane (e.g. along x) and through the first junction, then through F1, then through the second junction, and finally through the second part of F2 at out the other end. The current is always in the xz plane.
The prior art device discussed in reference to FIG. 1B is relevant only to magnetic tunnel junctions (MTJs), where three distinctions can be drawn. First, in the prior art structure, the edge is not perpendicular to the plane of the substrate (equivalently, the plane of the top surface of film F2). The result of the etching process is an edge that slopes about 55 degrees from the normal direction (i.e. from the y axis as defined in the disclosure). The portion of F1 that makes contact with the tunnel barrier is in a plane that is not perpendicular to the substrate. By contrast, the plane is more nearly parallel (35 degrees from parallel) with the substrate plane than perpendicular (55 degrees from perpendicular). Second, the remainder of the film F1 is in a plane parallel to the substrate. Third, the magnetization orientations of both F1 and F2 lie along an axis that's in the plane of the substrate (x axis as defined in the disclosure). It should be noted that other prior art also described junctions with edges that were not substantially perpendicular to the substrate plane.
A prior art reference describes a sandwich structure in which the flow of the bias current is promoted to be perpendicular to the plane of the films in a portion of the device, and parallel to the plane of the films in different portion(s) of the device. All films are in the xz plane and current flow is not homogeneous.
A prior art reference describes a magnetic edge tunnel junction with a single tunnel barrier. A bottom ferromagnetic layer, F2, is fabricated with an insulating film (I) on the top surface forming a bilayer (actually a trilayer because a third, thin layer is deposited between F2 and I). A photoresist mask and an Ar ion mill are used to expose one edge of F2, and a tunnel barrier is grown at the exposed edge. Finally a second ferromagnetic layer, F1, is deposited on top of the bilayer. The film F1 extends across the exposed edge of F2.